The present invention relates to a color television system in which at least one image signal is transmitted by means of amplitude modulation of a carrier.
There are many color television systems in which amplitude modulation is used for one or more image signals:
THE Y signal and two color difference signals in the NTSC, PAL, NIIR, ISA systems;
THE Y signal in the SECAM system.
It is known that during the transmission/receiving process, differential gain distortion may easily occur, or distortion may arise through the original signal becoming to some extent compressed and expanded. For example, such distortions are typical when using a radio link between a transmitter and receiver: when transmitting over a long distance distortion errors of up to 50% and beyond are not uncommon. Such distortions produce alterations in the resultant image at the receiver, which no longer reproduces faithfully the image at the transmitter in that the luminance level and, in the NTSC, PAL, etc. systems, the saturation values also, become different from the original values.
As is known, the PAL system is self-compensatory with regard to phase errors, which it transforms into saturation errors according to the cosine of the phase angle. Various complicated decoders have been proposed to correct this residual saturation error, which is at maximum of the order of 10-15%, (see for example U.K. Pat. No. 1,307,566). The differential gain error, on the other hand, for example on yellow, may be four times greater.
The object of the present invention is to remedy this state of affairs, providing a simple method of compensating for differential gain error.